[heap] Avoid map loads in LiveObjectIterator

Help the compiler a bit by moving the loads into the ctor.

Bug: 
Change-Id: I62deff0ee7252ea78dfec380e791ec958886005d
Reviewed-on: https://chromium-review.googlesource.com/533534Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45919}

src/heap/mark-compact-inl.h

@@ -50,9 +50,6 @@ void MarkCompactCollector::RecordSlot(HeapObject* object, Object** slot,
 
 template <LiveObjectIterationMode T>
 HeapObject* LiveObjectIterator<T>::Next() {
-  Map* one_word_filler = heap()->one_pointer_filler_map();
-  Map* two_word_filler = heap()->two_pointer_filler_map();
-  Map* free_space_map = heap()->free_space_map();
   while (!it_.Done()) {
     HeapObject* object = nullptr;
     while (current_cell_ != 0) {
@@ -73,10 +70,7 @@ HeapObject* LiveObjectIterator<T>::Next() {
         // last word is a one word filler, we are not allowed to advance. In
         // that case we can return immediately.
         if (!it_.Advance()) {
-          DCHECK(HeapObject::FromAddress(addr)->map() ==
-                 HeapObject::FromAddress(addr)
-                     ->GetHeap()
-                     ->one_pointer_filler_map());
+          DCHECK(HeapObject::FromAddress(addr)->map() == one_word_filler_map_);
           return nullptr;
         }
         cell_base_ = it_.CurrentCellBase();
@@ -124,8 +118,8 @@ HeapObject* LiveObjectIterator<T>::Next() {
         // Do not use IsFiller() here. This may cause a data race for reading
         // out the instance type when a new map concurrently is written into
         // this object while iterating over the object.
-        if (map == one_word_filler || map == two_word_filler ||
-            map == free_space_map) {
+        if (map == one_word_filler_map_ || map == two_word_filler_map_ ||
+            map == free_space_map_) {
           // There are two reasons why we can get black or grey fillers:
           // 1) Black areas together with slack tracking may result in black one
           // word filler objects.

src/heap/mark-compact.h

@@ -192,17 +192,21 @@ class LiveObjectIterator BASE_EMBEDDED {
       : chunk_(chunk),
         it_(chunk_, state),
         cell_base_(it_.CurrentCellBase()),
-        current_cell_(*it_.CurrentCell()) {}
+        current_cell_(*it_.CurrentCell()),
+        one_word_filler_map_(chunk->heap()->one_pointer_filler_map()),
+        two_word_filler_map_(chunk->heap()->two_pointer_filler_map()),
+        free_space_map_(chunk->heap()->free_space_map()) {}
 
-  HeapObject* Next();
+  V8_INLINE HeapObject* Next();
 
  private:
-  inline Heap* heap() { return chunk_->heap(); }
-
-  MemoryChunk* chunk_;
+  MemoryChunk* const chunk_;
   MarkBitCellIterator it_;
   Address cell_base_;
   MarkBit::CellType current_cell_;
+  Map* const one_word_filler_map_;
+  Map* const two_word_filler_map_;
+  Map* const free_space_map_;
 };
 
 class LiveObjectVisitor BASE_EMBEDDED {